The present invention relates to a thyristor which can be turned off by a negative gate current (gate time-off or GTO-thyristor) having an npnp structure including a cathode side n-type emitter zone, a p-type control base zone, an n-type main base zone which, at least in partial regions, changes toward the anode side into a higher doped n.sup.+ -type partial zone, and an anode side p-type emitter zone, and wherein the cathode side n-type emitter zone and the anode side p-type emitter zone are each divided into a plurality of emitter strips, with each cathode side emitter strip being opposed by two anode-side emitter strips which are positioned so as to overlap the respective edges of the respective cathode-side emitter strip.
It is known that the turn-off of a GTO-thyristor by a negative control gate current causes the current to be concentrated underneath the center of the emitter strip on the cathode side (IEEE Trans. Electron. Devices ED-13, July 1966, page 590).
This current constriction during thyristor turn-off can be counteracted in that the p-type emitter zone at the anode side interrupted by the main n-type base zone in strip-shaped regions centered underneath the associated n-type emitter strips on the cathode side and the n-type main base zone is there short-circuited with the p-type emitter zone, as shown, for example in Federal Republic of Germany DE-OS No. 2,538,042. With such anode shorting, it is additionally possible to suitably set the effective current gain factor .alpha..sub.pnp of the partial transistor on the anode side and thus set the turn-off characteristics without having to dope with recombination centers.
FIG. 1 shows the prior art configuration of a unit cell or element of such a GTO thyristor, with the cell being composed of a semiconductor body having an n.sup.+ -type emitter strip 1 at the cathode side, a p-type control base zone 2, an n-type main base zone 3 including a highly doped n.sup.+ -type partial zone 3a adjacent the surface plane or major surface of the semiconductor body at the anode side, and two spaced p.sup.+ -type emitter strips 4 at the anode side, which are disposed beneath and overlap the respective edges of the emitter strip 1. The p-type control base zone 2 is in part brought to the surface plane of n.sup.+ -type emitter strip 1 i.e., the cathode side major surface of the semiconductor body. Between n.sup.+ -type emitter strip 1 and p-type control base zone 2 there is an n.sup.+ p junction J.sub.1 whose ends extend to the cathode side surface, between the p-type control base zone 2 and the n-type main base zone 3 there is a pn junction J.sub.2 and between the n-type main base zone 3 and the p.sup.+ -type emitter strips 4 there is an np.sup.+ junction J.sub.3 whose ends extend to the anode side major surface of the semiconductor body. The surface of the n.sup.+ -type emitter strip 1 is provided with a metal contacting layer 5 (cathode K) and with a current input terminal 6, and the exposed surface of p-type control base zone 2 is provided with a metal contacting layer 7 (gate G) and a control terminal 8. Finally, the anode side surface is provided with a metal contacting layer 9 (anode A) and a current input terminal 10, with the layer 9 contacting both the surface of the p.sup.+ -type emitter strips 4 and the partial zone 3a of the n-type main base zone 3 extending to the anode-side surface.
It should be noted that the actual GTO thyristor has a plurality of unit GTO cells or elements as shown in FIG. 1 provided in the semiconductor body with a common control base zone 2 and a common main base zone 3. The current carrying capacity of the GTO thyristor is determined by the number of unit GTO cells provided.
In the unit cell of FIG. 1, a non-regenerative n.sup.+ pnn.sup.+ transistor structure I is centered below the n.sup.+ - type emitter strip 1. This transistor structure I functions to prevent undue concentration of the anode current into this portion of the cell when the thyristor is turned off. To realize, as a result of the shorting of the anode side emitter 4, a suitable setting of the effective current gain factor .alpha..sub.pnp of the partial transistor formed by zones 4, 3, 2, and thus of the turn-off characteristics, without doping with recombination centers, the anode-side p.sup.+ -type emitter strips 4 must have a width which is less than the customary spacing between the cathode-side n.sup.+ -type emitter strips 1 so that p.sup.+ -type emitter 4 is also cut out in the center region underneath gate G. If the blocking, firing and turn-on characteristics are set to be comparable to GTO thyristors doped with recombination centers, GTO thyristors with anode shorting have smaller turn-off losses. A limitation for the maximum A limitation for the maximum turn-off anode current results from the fact that the transverse voltage drop in p-type control base zone 2 from the center of n.sup.+ -type emitter strip 1 to the edge is at most equal to the breakdown voltage U.sub.Br of the n.sup.+ p junction J.sub.1 between p-type control base zone 2 and n.sup.+ -type emitter strip 1. Accordingly, the following condition applies: EQU J.sub.TCM /L =*G.sub.off U.sub.Br /(.rho..sub.s b) (1)
where .rho..sub.s is the sheet resistance of the p-type control base zone 2 underneath the n.sup.+ -type emitter strip 1, b is the width of the n.sup.+ -type emitter strip 1, G.sub.off =J.sub.A /J.sub.G is the turn-off gain, and L is the gate and emitter edge length. If the vertical current density is homogeneous, the factor * equals 4, but drops in the limiting case of extreme current constriction during turn-off to a value of 2.
It has now been found that GTO thyristors are generally destroyed already when significantly smaller currents are turned off than calculated according to condition (1). Additionally, the anode current which can be turned off without destruction greatly decreases with increasing voltage rise rate dU/dt during turn-off, which is not expressed in condition (1). The voltage rise rate is greater, the smaller the capacitance of the capacitor of the usually employed turn-off relief circuit (snubber capacitor). This is applicable, in particular, for the GTO thyristor of FIG. 1 which employs the above-mentioned measure for reducing current constriction.
It has also been found that in the GTO thyristor of FIG. 1 the maximum turn-off anode current during turn-off against high voltages and at high dU/dt is reduced because the electrical field occurring in the non-regenerative transistor region I, indicated in dashed lines, of the GTO thyristor are too high. These high fields are generated because, with the voltage already increased, the current density j in this transistor region I is still very high, for example 10,000 A/cm.sup.2, since current concentration is not completely eliminated even by shorting of the anode side. In the anode-side portion of the n-type main base zone 3 below the n.sup.+ -type emitter strip 1, the current is conducted, almost exclusively by electrons. Thus the electron concentration is relatively high there, according to the following relationship: EQU n.gtoreq.j.sub.n /(qv.sub.n) (2)
where q is the elementary charge and v.sub.n the saturation velocity of the electrons. Already at j=1,000 A/cm.sup.2, n is thus at least 6.multidot.10.sup.14 /cm.sup.3. Since the electron charge is compensated neither by the donor doping concentration N.sub.D +, which is, for example, 7.multidot.10.sup.13 /cm.sup.3, nor by holes, the electron current results in a high negative space charge. According to the Poisson equation EQU dE/dx=.rho./.gamma..gamma..sub.o ( 3)
this results in a rise of electrical field intensity toward anode A, in contrast to the stationary case reached after turn-off. As can be seen from FIG. 1, space charge zone R in non-regenerative transistor region I is therefore located in the vicinity of the n.sup.+ n junction between anodeside n.sup.+ -type zone 3a and n-type base zone 3, while in the adjacent regions, as after turn-off in general, it is located in the vicinity of pn-junction J.sub.2 because these regions carry only little current now and additionally in part have a four-layer structure.
Since, at high currents, the negative space charge is significantly greater than the positive space charge in the stationary case EQU -.rho..sub.dyn =qn&gt;&gt;qN.sub.D +=.rho..sub.stat,
the negative field intensity gradient and thus the maximum value of the field intensity at a given voltage is substantially greater than in the stationary case. Thus, the critical field intensity E.sub.cr leading to an avalanche breakthrough is reached during turn-off already at relatively low voltages. In a GTO thyristor according to FIG. 1, this results in a limitation of the turn-off current and of the voltage rise rate dU/dt during turn-off. A reduction of the mutual spacing between p.sub.+ -type emitter strips 4 results in partial compensation of the negative space charge by holes in the center region and thus to a reduction of excess field intensity. However, the current constriction in this region then becomes greater again. In particular, another advantage is lost during turn-off; namely that of a position of the anode-side p.sup.+ -type emitter strips 4 shifted greatly toward the gate G. This advantage resides in the fact that the injection of holes through the p.sup.+ -type emitter strips 4 when a negative control current is applied is prevented more quickly. In the short-circuited diode structure composed of p.sup.+ -type emitter strips 4, n-type main base zone 3, n.sup.+ -type partial zone 3a and anode contact 9, a short-circuit current then flows in the reverse direction through p.sup.+ -type emitter 4 and contact 9 to n.sup.+ -type partial zone 3a. This current component, which makes no contribution to the external anode tail current, instead contributes to the removal of the charge carriers, i.e. accelerates the turn-off process.
European Patent Application EP-OS No. 0,066,721, published Dec. 15, 1982 teaches increasing the permissible dU/dt value during turn-off, i.e. reducing the required RCD snubber circuit, without the maximum turn-off current and the sustaining voltage against which the turn-off can be made having to be reduced. To accomplish this, the thickness of the n-type main base zone is increased in the prior art GTO thyristor. However, a significant increase in the thickness of the n-type main base zone has the drawback that the tail current and the tail time, and thus the switching losses, are much higher compared to a GTO thyristor operated with a snubber.